Heatsealable seal and food package utilizing same



July 5, 1966 J. E. SMITH 3,259,507

HEA'ISEALABLE SEAL AND FOOD PACKAGE UTILIZING SAME BASE SHEET INTERFACE B LAYER A INTERFACE c ADHESIVE Fig.l

INVENTOR James E. Smith Attorney J. E. SMITH July 5, 1966 HEATSEALABLE SEAL AND FOOD PACKAGE UTILIZING SAME Filed Feb. 4, 1963 2 Sheets-Sheet 2 Fig. 13

Fig.l2

INVENTOR James E Smith Attorney United States Patent 3,259,507 HEATSEALABLE SEAL AND FGOD PACKAGE UTILIZING SAME James E. Smith, Hayward, Califi, assignor to Crown Zellerbach Corporation, San Francisco, Calif., a corporation of Nevada Filed Feb. 4, 1963, Ser. No. 255,783 11 Claims. (Cl. 99-172) This invention relates to seal constructions and to articles, such as labels, adhesive tapes, etc. having adhesive coatings, and the object is to provide constructions of such films or coatings having novel physical or mechanical relationships with each other whereby to produce advantageous sealing and delaminating properties, particularly for use in packaging food products, such as bread, in flexible wrappers, the folds of which are sealed together.

Within the contemplation of this invention are seals of any type wherein a base sheet is coated with adhesive and provides a supplemental sheet for application to a substrate, to which the base sheet is adhered by the adhesive, and wherein it is desirable to later remove the base or supplemental sheet in order to break the seal. These seals are of particular utility for use in sealing a package closure where it is desirable to regain access to the interior of the package without tearing or ripping the packaging material or the seal itself. Exemplary of these closures are fin-type package closures around which an adhesive coated base sheet may be sealed in a saddle configuration and box closures where the flaps of the box may be sealed together and to the box proper by adhesive coated tapes. Also, removable top closures for containers such as individual jelly servers provides a good example.

A particularly important object of the invention is to provide a novel coated sheet, or, in the vernacular of the industry, a label, for application to the folded over ends of a package, as for instance a bread package, to seal the overlapped flaps securely but which may easily be peeled from the end of the package to open it without tearing either the label or the package material so that the undamaged package may be used as a bag to store the contents not removed right away.

Patently, in packaging or in other adhesive seal applications, it is essential that a good bond be obtained between the surfaces adhesively sealed together. For example, the quality of package end seals is measured by the industry by their ability to cause fiber tear of either or both the package flap or the label before delamination of the adhesive takes place when the label is pulled from the package. In contradiction to this, particularly in packaging with flexible films or web materials such as paper, polyethylene, polypropylene or cellulose films, it is often desirable to be able to break the seal open at a later time without tearing the package or the seal.

In the past, mean attempts have been made to resolve these conflicting goals but these attempts have been for the most part unsuccessful. For instance, various combinations of base materials with adhesives have been devised wherein the bond between the adhesive and the object-to which the seal is affixed (substrate) is weaker than the internal strength of either the base sheet of the seal or the substrate so that the adhesive will rupture and/or (lelaminate before either the base sheet or the substrate tears or breaks. While certain individual combinations have been found to be workable, the inherent difiiculty is that these combinations are few and application of the coated sheet to other than the particular compatible bases will result in an inadequate seal or even no seal at all. Or, at the other extreme, a fiber tearing seal may result. In addition, even a slight variation in 3,259,507 Patented July 5, 1966 the conditions of application (pressure, temperature, etc.) may results in an inadequate or, at the other extreme, a fiber tearing seal.

Such limitations prevent wide-spread commercial use of these combinations because in industry it is of vital importance to use adhesives which operate satisfactorily and give a secure bond with numerous base materials under a wide range of temperature, pressure and other conditions of application.

Other attempts at solving this problem have included seal designs where a portion thereof has been left uncoated with adhesive to provide a pull tab or a central tear-out section. However, these constructions did little to facilitate opening of the wrapper since in either case it was essential to have a fiber tearing seal across the folds or lapped marginal areas to be sealed.

Accordingly, it is the primary object of the present invention to provide scalable constructions which, when applied to a wide range of substrates, are uniformly rupturable or delaminable, yet which are strong and durable and which may utilize a wide range of adhesives in preparation thereof. It is a further object to provide methods of preparing such novel scalable constructions and to provide packages with these novel seals.

The invention will be well understood from the following description in which various illustrative examples are given and which outlines the principles of the invention in such a manner that those skilled in the art may apply those principles to the selection and compounding of other specific constructions.

A sketch of the subject matter at this point will facilitate understanding of the detailed description which follows. This sketch, however, should not be considered as a full disclosure or as a discriminating definition. The subject matter can best be visualized by reference to FIG- URE 1 of the drawings which represents, diagrammatically, the cross section of a scalable construction of this invention.

Previously, sealing sheets have taken the form of a simple coating of adhesive, usually heat scalable, directly applied to a base sheet, which is usually constructed of paper. It is contemplated in the present invention to interpose between the base sheet and the adhesive a layer, termed for convenience layer A, at which layer delamination will occur in some fashion when the base sheet is pulled from the substrate to which it is afiixed through the adhesive. To function most desirably in accordance with this invention, the delaminability afforded by layer A is such that the base sheet 1 may be separated from the adhesive layer 3 (which is left on the object to which the seal is aflixed) before the base sheet tears or the substrate is torn or otherwise damaged. Stated in another fashion, the bonds holding the base sheet to the adhesive should desirably be less than the internal bond strength of the base sheet and/or the material of the substrate to which the seal is affixed and less than the strength of the bond between the adhesive and the substrate.

While it should first be noted that the following categories are not necessarily all-inclusive of seals in accordance with this invention, it is helpful for complete understanding of the invention to observe that the construction of layer A may fall within these types:

(1) Two adjacent coatings which develop only a relatively weak bond between one another, one of which forms a good bond with the base sheet and the other of which forms a good bond with the adhesive;

(II) A release coating which bonds relatively well to the base sheet but which forms only a weak bond with the adhesive;

(III) A coating of material which is weakly bonded to the base sheet but which forms an adequate bond with the adhesive;

(IV) A coating having a weak internal bond strength which will split internally when the base sheet is pulled to delaminate it from the adhesive.

As previously pointed out, the seals of this invention have particular utility in packaging applications, particularly for the packaging of bread. Bread is packaged in a variety of heat-scalable protective sheet materials such as waxed paper, regenerated cellulose film bearing a nitrocellulose .heat sealing coating, regenerated cellulose film bearing a modified polyvinylidine chloride (Saran) coating, polyethylene film, glassine, material foil, rubber hydrochloride film and polypropylene film. In the packaging of this commodity, either by hand or by automatic wrapping machinery, a sheet of the packaging material is first wrapped around the lateral surfaces of the bread and the opposite edges of the sheets lap-sealed together usually on the bottom of the loaf so that the wrapper forms an open tube enclosing the product. The wrapper is then folded down to cover the ends of the loaf in a series of overlapping end folds which are generally sealed in this position by heat and pressure. In general, these overlapping seals are relatively weak and have a tendency to pop open, exposing the product, and are quite irregular in formation so that they detract from the overall appearance of the package. Thus, it has been the standard practice in the industry to seal these end folds more securely by applying over the end folds a supplementary sheet as an end seal or label coated with a thermoplastic com position which, by the application of heat and pressure, becomes firmly bonded to the wrapping material, thereby strengthening the end closure and adding to the attractiveness of the finished package. In order to obtain a satisfactory end seal, the adhesive on the supplementary sheet must form a secure bond to the particular wrapping material used.

Thus, the optimum desirability is the use of an adhesive which will give a secure bond to the many types of wrapping materials used as described above and which will obtain this bond under a wide variation in applicating conditions, i.e. temperature and pressure. The problem of finding an adhesive which will furnish these optimum qualities and yet which will delaminate from the wrapping material without a fiber tear becomes readily apparent. However, through the present invention, this result is indirectly achieved by the use of one of a number of existing all-purpose satisfactory adhesives with an intermediate delarninating interface between the adhesive and the base sheet of the seal.

With the seal constructions of this invention it is found that there is an unexpectedly high resistance to shear I stress in the plane of the seal over that which would normally be expected where there is a relatively poor bond between the adhesive and the base sheet. For instance, when scalable constructions of this invention have been applied to the end folds of a bread packaging having a paper, polyolefin film or cellophane wrapper and the package is thrown violently against a flat surface, failure is found to occur not at the seal but in the wrapper itself, usually around the periphery of the package near one end thereof. Nevertheless, the base sheet may thereafter be peeled easily from the end folds of the destroyed package.

Referring now more fully to the drawings, FIGURE 1, as previously indicated, represents diagrammatically, a cross section of a laminate construction useful in describing the present invention in broad outline;

FIGURE 2 is a plan view of the coated face of a sealable sheet or label for sealing the end of a package, illustrating the pattern of adhesive coating applied thereto to provide a pull tab band;

FIGURE 3 is an isometric view of a seal or label roll, embodying the invention, from which individual labels may be cut;

FIGURE 4 is an isometric view illustrating an article, such as bread, wrapped and end sealed by a label having coatings as provided in this invention;

FIGURE 5 is a fragmentary vertical section taken along lines 5-5 of the package illustrated isometrically in FIG- URE 4 showing the relationship of the label to the end of the package to which it is adhered through the coatings thereon, the thickness of the label being exaggerated for purposes of illustration.

FIGURE 6 is a sectional view of the label of FIGURE 2 along line 6-6 with the thickness exaggerated to illustrate a preferred arrangement of the coatings according to this invention;

FIGURE 7 is an isometric view illustrating the package of FIGURE 4, with the end seal base sheet being peeled therefrom;

FIGURE 8 illustrates the package of FIGURE 7 with the end seal base sheet completely peeled off with the residue layer of the seal remaining on the end folds of the package and the outer most end fold pulled out, rupturing the residue layer;

FIGURE 9 illustrates the package of FIGURE 7 with the end seal base sheet completely removed and the end folds completely opened to expose the contents, the residue layer having been sheared at the junctures of the end folds;

FIGURE 10 illustrates the package of FIGURE 7 completely reclosed after removal of a portion of the contents;

FIGURE 11 is an enlarged fragmentary, sectional view further illustrating the construction of FIGURE 6 at the left adhesively secured to a substrate such as a wrapping sheet and at the right after separation of the base sheet from the substrate;

FIGURE 12 is an enlarged fragmentary, sectional view illustrating the seal construction of FIGURE 5 at the left adhesively secured to a substrate such as a wrapping sheet and at the right after separation of the base sheet from the substrate;

FIGURE 13 is an enlarged fragmentary, sectional view illustrating another construction in accordance with the invention at the left adhesively secured to a substrate such as a wrapping sheet and at the right after separation of the base sheet from the substrate;

FIGURE 14 is an enlarged fragmentary, sectional view illustrating another construction in accordance with the invention at the left adhesively secured to a substrate such as a wrapping sheet and at the right after separation of the base sheet from the substrate,

While the scalable constructions of this invention may be utilized for many purposes in a tape of sheet form, for application as an end seal for flexible packages, they conveniently take the form of a modified square as illustrated in FIGURE 2 of the drawings. These are formed from a roll of the sealing material in strip form, such as that depicted in FIGURE 3, prepared as described, for instance in U.S. Patent No. 2,984,342 to Smith.

In commercial operations for sealing the end folds of a bread wrapper as illustrated in FIGURE 4, the seals are automatically cut from the sheet (as at cut lines 11 in FIGURE 3) as it is unwound from the parent label roll and are automatically applied and heat sealed to the wrapper end folds by well-known bread wrapping and end label applying equipment such as is disclosed in U.S. Patent No. 2,349,317 to Weida and U.S. Patent No. 2,409,065 to Pohl.

BASE SHEET Quite commonly, the seals are formed with notched corners 5 but they may be rectangular in shape or any other suitable shape. Preferably, for application of the seals to the ends of wrappers, the base sheet is of cellulosic sheet material, such as paper. Usually, the paper is of about 35 to 45 pounds per ream of good grade sulphite paper. Alternatively, base sheet 1 may be composed of regenerated cellulose, paper laminated to metal foil, polyolefin film or like material or other flexible materials having an appreciable internal strength, both with respect to shear and tensile stresses.

ADHESIVE Adhered to the base sheet 1 through an intermediate delaminating A layer (that is an intermediate layer providing a readily relaminable interface described in detail hereinafter) is a surface layer of adhesive composition which, preferably, is normally non-tacky but which may include compositions activated by heat, pressure or action of a liquid for application of the sealable construction. The main criteria for choice of adhesives are, first, that it desirably should create an effective bond with the surface (substrate) to which the sealable construction is applied and with the intermediate delaminating layer (except for those constructions in accordance with this invention where delamination is to take place at the interface between the adhesive and the intermediate layer, in which case the adhesive desirably is of a nature such that a weak bond is established between it and the intermediate A layer). Second, the adhesive preferably is chosen with respect to the A layer so as not to cause fusion at the delaminating interface to destroy the relarninating qualities of the seal.

Some adhesive compositions, particularly of the heat activatable or pressure sensitive type, pose a particular problem of compatibility with the A layer in that they contain plasticizers, usually for the purpose of giving residual tackiness to the adhesive. Heat activatable adhesives of this type are exemplified by those adhesives described in US. Patent No. 2,462,029 to Perry dated February 15, 1949. In this case it is desirable to choose the A layer with regard to its dissolvability in any component of the adhesive such as the plasticizer. If the A layer component (or components) are dissolvable in the plasticizer, fusion of the delaminating interface may possibly occur. Therefore, it is preferable (although not necessarily critical) to select an A layer of a material that is not dissolvabl'e in the plasticizer f the adhesive.

A similar A layer interface fusion problem may arise of the adhesive is applied to the A layer in a solvent medium that is a common solvent for the materials at the delaminating interface of the A layer. It is preferable, although not always critical, to use an alternative method of application, e.g. using a different solvent or applying as a hot melt.

Use of a solvent activated adhesive presents a problem similar to that in applying an adhesive in a solvent medium. If the solvent activated adhesive is applied to the A layer in solution (or dispersion), the solvent (or dispersant) must not be such a strong solvent for the materials at the A layer so as to fuse the delaminating interface by solvent action. Although it has been found not always necessary to use a solvent in which the materials of the A layer are insoluble, it is preferable to do so.

On the other hand, in applying a solvent activatable adhesive, to obtain a good bond between the adhesive and layer A, it is sometimes necessary to select the adjacent A layer material and the dissolved adhesive which is applied thereto so that the A layer material is at least partially soluble in the solvent.

In every instance, suflicient adhesive should be applied to obtain a good bond with the A layer and with the desired substrates. In the case of a solvent activatable adhesive, preferably 12 to 16 pounds per ream is applied.

When applied in the preferably bump pattern (as hereafter described), a heat sensitive adhesive may be between 8 to 12 pounds per ream. However, if a continuous coat is desired, a greater amount, up to 20 pounds per ream, may be preferable. In the case of a pressure sensitive adhesive, to pounds per ream is preferable.

' Although economy mitigates against a heavier adheas provided herein are adhered to a substrate through- I out only a portion of the adhesive coated area so that delaminating at the A layer may not be started easily by peeling at a lateral terminal edge of the adhesive area, it becomes necessary to cause the adhesive layer to tear apart at the line where adhesion to the substrate ends in order to start delaminating at the A layer. In such instances, it may be preferable to use a minimal thickness of adhesion or to choose an adhesive which is weak in tear strength to minimize the chance of tearing the substrate or base sheet due to the pull required to tear through the adhesive.

The adhesive coating may be applied as a thin, even film to the base sheet as is shown more clearly in FIG- URE 6, or, preferably, may be applied as a series of minute, discreet islands or mounds of coating, as shown at 36 in FIGURE 13, by the use of an engnaved or knurled applicator roll. The manner in which these projections may be formed and the method of their application is described more fully in US. Patent No. 2,984,342 to Smith.

In the embodiment of FIGURE 2, the base sheet is free of coating along a relatively wide transversely extending strip or area 8 adjacent edge 9 and between opposite edges 10 of the seal. This provides an unsecured band forming an adhesive free pull tab for facilitating removal of the label after it has been affixed to an object. This pull tab section is not essential to constructions of this invention since an adhesively secured edge may be peeled back Without much difiioulty. It is quite desirable, however.

It will be noted from FIGURE 4 that the wrapper sheet about bread loaf 21 is conveniently wrapped with its end folds disposed so that top fold 22 underlies bottom fold 23 and that fold 23 is the outermost fold. The central adhesive coated region 24 of the label is 'heat sealed over exposed portions of both folds 22 and 23 and of the side folds to hold them together securely.

Also illustrated in FIGURE 5 is delaminating layer 2 interposed between adhesive layer 3 and base sheet 1, it should be clear from later discussion that it need extend over only the areas of base sheet 1 covered by adhesive, i.e.,- area 24. However, as a matter of commercial expediency, it is simplier to apply uniform coverage of the A layer films.

To illustrate the operation of the seals of this invention is one of their most important uses, as a package end seal and the unique advantages obtained thereby, references is made to FIGURES 7 through 10. To open the sealed package in FIGURE 7, the seal is grasped at pull tab 8 and pulled downwardly and outwardly. Because of the low bond strentgh at delaminating layer 2, the base sheet 1 easily peels from the end folds of the package. After base sheet 1 is completely peeled from the end folds as in FIGURE 8, the end folds remain slightly sealed together by the residual adhesive layer 3 (and, in some embodiments of the invention, a portion of delaminating layer 2 which remains adhered to the ad hesive layer). The residual layer (or layers) is so weak in tear strentgh that the end folds may readily be pulled outwardly to tear the layer (or layers) at the edges of the exposed end folds so that the package may be completely opened as in FIGURE 9, to allow access to the contents of the package such as the slices of bread 25.

At this point, the pack-age has been easily opened without ripping or tearing the base sheet of the seal or the end folds of the package so that, after a portion of the contents have been removed, the end folds may again be closed to leave an unbroken, relatively airtight and attractive package as in FIGURE 10. The package is shown with the end folds refolded in the original manner. Alternatively, particularly in the case of the more flexible wrapping materials, the open end of the package may be twisted about itself with the fingers to form a closed bag. Thus, through this invention, packages, such as for containing bread or other comestibles, may be opened, stored and reused in as good a condition as they were originally.

TEAR STRENGTH OF RESIDUAL'LAYERS As noted in the foregoing general description, in seals created in accordance with this invention, a residual layer of adhesive or a layer of adhesive and all or a portion of the A layer, depending upon the particular A layer construction, will remain on the substrate after the base sheet is peeled therefrom. These residual layers must be sufficiently weak so that they may be torn apart to open the closure. For example, the residual layers of a seal for the end folds of a package should be weak enough to allow easy parting of the folds at their junctures without tearing. To function properly, the residual layers must tear apart before the substrate, as for example a wrapping sheet, is torn. Thus, it is desirable to provide an adhesive layer (and an A layer where the A layer remains on the substrate) light or thin enough to tear relatively easily compared with the strength of the substrate. In the usual case the economic considerations dictating the minimum possible coating weights commensurate with the desired good adhesiveness of the seal and ready peelability minimize this problem since the resulting residual coatings are quite thin and weak.

PEEL STRENGTH Seals of the present invention are provided with an intermediate layer (between the adhesive and the base sheet) having an interface at which the bond is sufficiently weak so as to delaminate when the base sheet is peeled from the substrate to which it is bonded through the intermediate layer and the adhesive, i.e. an intermediate delaminating layer. For definiteness and to assist in the choice of materials for construction of the intermediate delaminating layer for a particular application of the delaminating eifect which is charatceristic of the invention, standard procedures for determining practical bond strength (hereinafter called peel strength) at the delaminating layer are desirable. Before proceeding with a detailed description of these various constructions to illustrate the invention, such procedures will be described.

Sealable constructions in sheet form are prepared in accordance with this invention and trimmed to a 3 X 6 inch strips, the adhesive coated area extending entirely over each strip.

These strips are each superimposed over a 3 x 6 inch strip of sheet material used as a substrate. In the measurements related herein, a clay coated, supercalendered bleached sulphite paper of a 45 pound ream basis weight was used as a substrate. However, any substrate may be used which bonds well to the adhesive employed. In general, the more resistant the substrate is to internal delamination or fiber tear, the wider range of delamination or peel strength values may be measured, since failure in the substrate (or base sheet) is the upper limit for determining the peel strength at the delaminating interface.

Where the adhesive used in the construction is of the heat activatable type, the strips, thus assembled, are sealed together as by pressing at 20 pounds per square inch pressure for two seconds between the jaws (heated at 350 F.) of a Sentinel heat sealer, Model 12-A. After heat activation, the resulting seal may be cooled while being maintained under pressure by weighting it down with a cool metal bar.

For pressure sensitive adhesive constructions, the assembled strips may be pressed with a roller or with the fingers to obtain a uniform adhesive seal. The same procedure may be used for solvent activated adhesive constructions except that, prior to assembly of the strips, the adhesive is activated by moistening it with an activating solvent, as for instance, water. The seal, after pressing, should be air dried to remove the residual solvent.

It should be noted that the exact procedure above is preferable but not critical as it is essential only that a secure adhesive bond be obtained with the substrate, of course, without fusion of the A layer delaminating interface.

The resulting seal is trimmed to obtain a l x 6 inch specimen strip. To determine delaminating or peel strength, the substrate is laid on a fiat surface and afiixed thereto by adhesive or the like and the base sheet at one end of the strip attached to a strain gauge (a short portion of the strip may be peeled back to serve as a tab for attaching the strain gauge) and pulled back along itself at an angle of 180 at a rate of 12 inches per minute by the gauge so that the force required to cause delamination may be observed directly from the gauge and reported as peel strength in units of grams per lineal inch of seal per pendicular to the direction of delamination. This determination may be made, for instance, on an Egan Slip Tester, Catalog No. 225, To obtain a reliable determination, at least three seal specimens of a particular construction should be tested.

If the delaminating strength at the A layer is greater than the internal strength of the base sheet or the substrate, undesirably, fibers in either or both will tear rather than delamination taking place at the proper delaminating interface. For example, with the most preferred base sheet material, paper, of the quality previously described, the peel strength at the A layer must be less than the internal strength of the paper, otherwise fiber tear results.

It is thus to be noted that the delaminating strength of the particular base sheet used in the test imposes an intrinsic limitation on the determination of the peel strength of a layer construction. Paper has been chosen for convenience and also by preference, unless otherwise specified, in the test data which follows.

It will be demonstrated at a later point that the A layer construction may have so high a peel strength that a paper base sheet will fail before delamination occurs; yet with a stronger base sheet, delamination may be obtained at the A layer preferentially to failure of the base sheet. It should thus be kept in mind that particular A layer constructions may be limited in their use to a base sheet having a certain level of delaminating strength but that the principles of this invention are not arbitrarily limited to a certain maximum delaminating strength of the A layer. The selection of the base sheet and A layer must be based on their relative delaminating strength. Patently, the peel strength of the A layer must be less than the internal delaminating strength of the base sheet.

The myriad of constructions possible in the practice of this invention provide a wide range of A layer peel strength to choose from. The most satisfactory peel strength is obtained by balancing the ease of peeling of the base sheet of the seal and the secureness of the seal against delainination through abrasion, scuffing, etc.

While satisfactory results may be obtained over a broad range, a peel strength of 60-70 grams per inch according to the above test procedure has been found to be the most satisfactory for package end seals. On the other hand, end seals having 20 grams per inch or less peel strength are easily susceptible of being accidentally peeled off in handling and are thus not generally desirable for this specific use.

While the manner in which the A layer is prepared on the base sheet and the conditions under which the seal is made both have an effect on the peel strength (particularly as hereinafter related for Category III), generally the selection of the particular coating or coatings comprising the A layer, the base sheet and the adhesive, determine the delaminating strength of the particular construction.

The materials that may be employed in the formation of layer A are, generally, and film-forming material which has an interface characterized by a mechanically disruptable bond weaker than the internal delaminating strength of the base sheet and the flexible packaging material to which it is applied, and which is nonfusible at the activation temperature of the heat sealable adhesive. Exemplary of such layer A materials are natural resins such as shellac and Vinsol, modified natural resins such as the esters of rosin acids, modified natural polymers such as the esters and ethers of cellulose and chlorinated natural rubber, synthetic polymers such as vinyl copolymers, polystyrene, acrylic esters, polyvinyl alcohol, organopolysiloxanes, polyethylene, polypropylene, polyamides, butyl rubber, and others as will be set forth hereinafter.

In the following examples, conveniently arranged according to particular categories for ease of understanding, are illustrated with diverse materials a number of constructions that may be usefully employed in accordance with the principles of this invention. A number of these embodiments represent species of the present generic invention. It is to be understood that there are many other possible embodiments within the practice of the present invention in addition to those described in detail hereafter that may come to mind with or without the exercise of invention.

(1) DELAMINATION BETWEEN TWO COATINGS COMPRISING LAYER A One of the preferred categories of seal constructions in accordance with the present invention is illustrated by FIGURE 6, wherein the layer A as previously described, comprises a double coating of dissimilar film forming materials 2a and 2b interposed between base sheet 1 and adhesive 3. Layers 2a and 2b are chosen for their ability to form a weak bond with one another when one is coated on the other. One of these coatings, in the example coating 20, should bond at least weakly to base sheet 1 and the other coating in this example coating 2b, should bond at least weakly to the adhesive 3.

When this type of seal is adhered to a substrate, such as, for example, the wrapping sheet 30 shown at the left in FIGURE 11, base sheet 1 may be removed from its aflixed position by pulling it away from sheet 30, whereupon delamination readily occurs at the interface of coating 2a and 2b as shown at the right.

The amount of material required to create coating 2a is dependent not only upon its composition but also upon the type of base sheet, i-.e. the base sheet porosity and absorptive qualities, since generally the surface of the base sheet will absorb a certain amount of the A layer material applied which would otherwise form a portion of the continuous film over the base sheet. Also, the manner in which the coating is applied has a great influence upon the continuity thereof. The better the equipment used, the less material is required to perfect an adequate continuous coating. This consideration applies to all types of A layer coatings in accordance with this invention.

The continuity of the 2a coating is important in order to prevent any significant amount of strikethrough of coating 2b when it is applied. Any significant amount of strikethrough of the 2b layer to the base sheet may possibly result in a fiber tearing bond rather than the delaminating *bond desirable at the 2b-2b interface. In general, 1.0 to 3.0 pounds per ream or more of the 2a coating is satisfactory.

Similarly, it is important that layer 2b be a continuous coating formed over layer 2a of a sufficient thickness, with respect to the adhesive coated thereover, to prevent strikethrough of the adhesive to the delaminating interface, both when the adhesive is applied thereto and when the adhesive is activated for affixing to the substrate. If strikethrough occurs to a significant degree, the effect may possibly be a v 20%- chlorinated natural rubber seal not unlike the prior art, i.e., a direct adhesive bond between the substrate and the base sheet.

In general, 1.0 to 3.0 pounds per ream or more of coating is satisfactory for layer 2b, although weights as low as 0.4 to 0.5 pound per ream have been found to be adequate in some instances, when applied with commercial coating equipment.

At very low coating weights of both A layer coatings, the peel strength resulting may in many instances greatly increase due to a certain amount of strikethrough, although reasonably satisfactory peel still results.

Example 1a The following is an example of a method of preparing a seal of a specific construction presently most preferable within this category.

A web of base sheet stock is passed through a rotogravure press to apply the necessary A layer coating. The base sheet stock consists of a 48 pound per ream basis weight starch sized, supercalendered sulfite paper containing minor amounts of ground Wood and bleached kraft stock, the starch size constituting a 13 pound per ream coating containing a minor amount of latex binder (6 /2 pounds applied to each side).

The first coating applied is nitrocellulose. More accurately, the nitrocellulose is cellulose partially esterified with nitric acid and is often termed pyroxylin. For use in a lacquer, pyroxylin is modified, as by heating for a considerable time in slightly alkaline solution, to make it soluble in organic solvents.

A nitrocellulose lacquer, prepared according to the following formulation, is applied in a uniform coating to thebase sheet at a first rotogravure lacquer station at a coating Weight of 1.3 pounds per ream:

20% by Weight nitrocellulose (viscosity 18-25 c.p.s., ethyl acetate soluble and degree of nitration 11.812.2%) 74% by weight solvent (4 parts ethyl acetate to 1 part methyl cellosolve) 5% by weight tricresyl phosphate 1% paraffin wax (M.P. 143-150 F.)

(viscosity 5 c.p.s., degree of chlorination 67% by weight) toluen generated cellulose, polypropylene, polyethylene, waxed paper and Saran:

Percent Elvax 220 35 Elvax 250 10 Staybelite Ester #10 5 Staybelite F f 10 Piccoumaron 450-L 5 Microcrystalline wax melt point -16'5 F. 35

Elvax 250 is the trade designation for a high molecular weight ethylene-vinyl acetate copolymer having a vinyl acetate concentration by weight of polymerized vinyl acetate of 28%,

a tensil strength of p.s .i., a ring and 1 1 ball softening point of 252 F., a brittle point of less than 70 F. and a melt index of 15.

Elvax 220 is the trade designation for an ethylene-vinyl acetate copolymer having substantially the same vinyl acetate concentration (28.4%) but a higher melt index (150). For this copolymer grade the tensile strength is 460 psi, ring and ball softening point 192 F. and the brittle point is the same.

Staybelite Ester is the trade name for a thermoplastic glycerol ester of hydrogenated pine Wood rosin having an acid number of 8, Hercules drop softening point of 84 C. and specific gravity of 1.07 at 25 C.

Staybelite F is another trade designation for a thermoplastic hydrogenated wood rosin having a Hercules softening point of 75 C., acid number of 166, density at 20 C. 84 C. and specific gravity of 1.07 at 25 C.

Piccoumaron 450-L is a product which constitutes a thermoplastic polyindene resin having a softening point of 100 C., acid number of 1, specific gravity of 1.09, refractive index of 1.63 and melt viscosity of 175 C. of 1 poise.

Although the above composition is preferred, various components thereof may be omitted on their concentrations varied within broad ranges. For instance, satisfactory results may be obtained with petroleum wax having a melting point between 130 F. and 170 F. and the ethylene-vinyl acetate copolymer may have a weight content of polymerized vinyl acetate between 20% and 35%.

The above described composition is prepared by heating and blending the materials in a hot melt for application at the third rotogravure station.

As is described in detail in US. Patent No. 2,984,342, referred to previously, the rotogravure type applicator roll is provided with spaced apart grooves running obliquely and parallel from one end of the roll to the opposite end or a surface knurled pattern of some other type so as to apply a grid like dotted pattern of adhesive. As also described in the above patent, a back-up or impression roll is covered with spaced apart transversely extending sections of resilient covering materials, such as rubber, which provides uncovered sections therebetween.

coated area), leaving uncoated areas appropriately for providing the pull tab sections of the finished seals.

The sheet is cooled to solidify the adhesive and then fed through the usual punching and slitting machines for preparing rolls of end labels as depicted in FIGURES 2 and 3 of the drawings.

The seals thus prepared may be tested on commercial bread wrapping machines for sealing the end folds of the bread wrappers over the extremes of operating conditions encountered thereon which include heat applicator temperatures from 160F. to 550 F., wrapping speeds from to loaves per minute and variable application pressures. The individual seals are cut on the machine from a parent label sheet into individual seals as depicted in FIGURE 2 of the drawings heated and applied to the end folds with a moderate pressure. The end seals thus formed provide a strong closure for the braed packages with more than adequate resistance to delamination by scuffing, compression, and rough handling of the packages. The base sheet peels readily from the end folds of the package (carrying the nitrocellulose coating therewith) to leave a weak residue coating of adhesive and the chlorinated rubber coating. This residue coating is readily torn when the end folds are pulled outwards and the package opened with no damage thereto.

Other specific constructions falling within this category may be prepared using the above procedure to give the peel strengths indicated in Table I. These results are based upon the use of roughly 2 pounds per ream coating weights for each of the A layer coatings and the same heat activatable adhesive formulation in the same coating weight as for Example In. These results are also based upon use of the same base sheet as described in Example 1a. Determination of the peel strength of these formulations is in accordance with the previously described test procedure, wherein the peel strength for a given A layer combination is in terms of the force in grams per inch of seal perpendicular to the direction of tear at the delaminating interface between the coatings. Where fiber tear is obtained rather than delamination at the delarninating interface, the designation FT is used.

Table I A LAYER CONSTRUCTIONS a b c d e f g h i j k 1 Ex. First Coating Second Coating (Coating 2b) (Coating 2a) Ethyl 00% Vinyl Poly- Pcnta- Aro- Parlon Cell. Shellac NO C0poly- Vinsol Poly- Acryvinyl lyn 833 chem Bcckacitc 5 cps. N /10 Soln. Cell. iner Resin styrene loid Alcohol 455 1126 VAGH 1 Nitrocellulose 2 Ethyl cellulose FT '1. 3 60% Shellac S0ln Parlon 5 cps- Vinyl Copolyrne Vin ol Resin Polystyrene.

Acryloid FT F FT Polyvinyl Alc0hol. FT 1520 FT FT '1 FT, 15-30 15-30. Pentalyn 833 35-80-.- 200 -120" 70 -150.- 60-100-. 100-140.. 30-50. Arochem 455 FT FT- FT FT FT FT., 150-170.. FT. Beckacite 1126 100 FT 100-140. 30-60 130 -150. 150400" X (7590).

Internal delamination peel strengthSec Example 29.

The uncovered sections of the impression roll effect the uncoated sections on the base sheet web. A pick-up roll cooperates with the applicator roll by picking up adhesive from a molten liquid mass at a desirable temperature of 300 F. to transfer same in the form of a film to the knurled applicator roll.

The adhesive applicator roll applies adhesive in a dot pattern to the chlorinated ruber coating at a preferable coating weight of 10 pounds per ream (computed on the In Table I, the column entitled first coating represents the coating first applied to the base sheet and the columns under the general heading second coating represent the second coating applied to the first coating previously applied to the base sheet. A more comprehensive description of the various coating materials described in Table I follows.

The nitrocellulose coating of Example Series 1 and Series d is the same as that previously described in Ex- Percent Ethyl cellulose N-l 15 Dibutyl phthalate Toluene 80 Ethyl Cellulose N- is the ethyl ether of cellulose with an ethoxyl content of 47.5-49.0%. The degree of substitution of ethoxyl groups per anhydroglucose unit is from 2.42-2.53. Viscosity in a 5% solution of 80:20 ratio of toluenezethonal mixture is 10 cps.

The shellac coating in Example Series 3 and Series 0 consists of bleached shellac, a brittle, yellowish, resinous excretion of the insect Laccifer. To provide a lacquer for application of the shellac coat, a 60% solids solution of shellac in anhydrous ethyl alcohol is further diluted one for one with anhydrous ethyl alcohol.

Parlon S-5 has been previously described in Example In, as a particular chlorinated rubber composition. This same coating may be applied in the same lacquer formulation as indicated in Example 1a in every instance in Example Series 4 and Series d.

The vinyl copolymer of Example Series 5 and Series e constitutes a partially hydrolyzed copolymer of vinyl chloride and vinyl acetate having a vinyl chloride content of 91%. This material has a specific gravity of 1.35 and an intrinsic viscosity (determined in cyclohexanone at 20 C.) of 0.57. This material may be obtained under the trade designation VAGH. This vinyl copolymer is dissolved to 20% solution in methyl ethyl ketone to provide a convenient lacquer for application as in Example la.

The Vinsol resin of Series 6 and Series is a resinous product derived from that portion of the solid resinous matter extracted from southern pine wood which is insoluble in aliphatic hydrocarbon solvents. It is a dark colored, high melting, thermoplastic material, available from Hercules Powder Company under the trade name Vinsol, consisting of acidic phenolic constituents in the form of high molecular weight phenols, carbonyl substituted phenols, substituted phenyl ethers and polyphenols. It also contains acidic materials derived from resin acids and oxidized resin acids. High molecular weight neutral compounds derived from resinous and polymerized terpenes are also present along with some wax. This resin has an average molecular weight of 470, and acid number of 94, and a methoxyl content of 5.3%. Its density at 25 C. is 1.218 and it has a softening point of 120 C.

This resin is applied as a lacquer having a 45% Vinsol concentration in ethyl alcohol (95%).

For the polystyrene coating of Series 7 and Series g, a polymer of styrene having the following characteristics may be used: specific gravity 1.05; tensile strength 5500- 7000 pounds per square inch; distortion temperature 172- 176 F., and softening point 220240 F. This material may be obtained under the trade designation Styron 666. This resin may be applied in the following lacquer composition:

Percent Styron 666 2O Dibutyl phthalate 5 Toluene 75 The acryloid coating of Series 8 and Series 12 is a polymerized acrylic ester resin in the form of a waterwhite, transparent solid. It has a specific gravity of 1.18 and a viscosity of 225-340 cps. at 30 C. in a 40% toluene solution. This material is available under the trade name 14 Acryloid B-66. This coating may be applied in the following lacquer composition:

Percent Acryloid B-66 20 Dibutyl phthalate 5 Toluene 75 The polyvinyl alcohol of Series 9- and Series i may be prepared by incomplete hydrolysis of polyvinyl acetate, preferably from 86% to 89% hydrolysis. Such a partially acetylated polyvinyl alcohol may be obtained under the trade name Vinol PA-S. Vinol PA-S may be dissolved .in water to a 20% solution for application as a lacquer.

In Example Series 10 and Series j, the coating indicated as Pentalyn 833 is a phenolic modified pentaerythritol ester of rosin acids. This substance has an acid value of 23, a Hercules drop softening point of 182-188 C. and a specific gravity at 25 C. of 1.10. This resinous ester is furnished under the trade name Pentalyn 833. For application .as a lacquer, this material is formulated as follows:

Percent Pentalyn 833 30 Dibutyl phthalate 5 Isopropyl alcohol 42 Methyl acetate 20 Cellosolve acetate 3 The coating designated Arochem 455 in Series 11 and Series k is a polymerized modified ester of rosin acids, similar to abietic acids, having a melting point (mercury method) of 135-145 C. and an acid value of -90. This material is available under the trade name Arochem 455. For application of this coating, it is prepared in The coating of Series 12 and Series I, designated Beckacite 1126, is a modified phenolicester of rosin available under this trade name designation. It has a specific gravity of 1.06-1.11, a melting range of 302-320 F. and an acid number 12-18. A lacquer composition of the following proportions is prepared for application of this coating:

Percent Beckacite 1126 30 Dibutyl phthalate 5 Isopr-opyl alcohol 42 Methyl acetate 20 Cellosolve acetate 3 The above examples demonstrate that, where the same materials are used for the A layer coatings but the order in which they are applied to the base sheet is reversed, more often than not, different peel strength values are obtained. For instance, compare Examples If and 6d, where the Vinsol resin is applied as the first coating (layer 2a) internal delaimination of this particular base sheet occurs preferentially to delamination of the interface of the coatings. But,-where the nitrocellulose coating is applied first with the Vinsol coating thereover, delam-ination occurs preferentially at the interface of the coatings with a quite low peel strength of 20 grams per inch. Or, compare Example with Example 3 Where shellac comprises the first layer and Pentalyn 833 the second, a low peel strength of from 10-30 grams per inch results. Yet, where the coatings are reversed in Example 100, the peel strength is in the range of 80-120 grams per. inch.

This result is believed to be due to a numberocf factors, including the difference in the degree of solubility of the first coating down in the solvent vehicle for the second coating and the susceptibility of either coating to becoming fused .by the heat used to activate the adhesive when the scalable construction is sealed to a substrate. The second coating may be fused by the heat to some extent to the first coating. Also, the greater the solubility of the first coating in the solvent of the second coating, the more fusion there is of the coatings at their joint interface to increase the peel strength. It may be important, therefore, in many combinations, to dry the first coating thoroughly before applying the second coating in the mutual solvent in order to minimize the fusion of the delaminating interface.

In this regard it should again be noted that simply because the first coating is solu-ble in the solvent in which the second coating is applied does not necessarily mean that their joint interface will be completely fused. Examples 2g, 10k, 101 and 12 provide a good illustration of this, since, in each case, the same solvent media was used for application of both coatings yet an interface bond with desirable peel characteristics resulted.

To further demonstrate the manner in which the many variables may be manipulated to achieve the peel strength desired, refer to Example 6g, which construction has poor peel characteristics with the base sheet employed (50% fiber tear), and to Example 2g which construction has a low peel strength (-40 grams per inch). By adding to the Vinsol coating composition used in Example 6g, various concentrations of the ethyl cellulose as used in Example 2g, the peel strength of the construction (otherwise the same as in Example 6g) may be adjusted to obtain any intermediate peel strength between the peel strength of the Example 2g and 6g constructions.

For example, where the Vinsol coating is replaced with a coating resulting from the following coating composition, a peel strength in the range of 100 grams per inch results:

Percent Vinsol resin Ethyl Cellulose N-10 5 Dibutyl phthalate 5 Ethyl alcohol 60 In addition to the above constructions, the following specific constructions in this category may be prepared and tested in the same manner.

Example 13 Bleached sulfite supercalendered paper, basis Wei ght pounds, is coated first with a 5 pounds per ream coating of low density (0.923 sp. gr.) polyethylene, having a melt index of 4 and a softening point of 200 F., by extruding a molten film of the polyethylene on the paper surface. Any other coating method which gives a secure paperpolyethylene bond may be used. Then, the polyethylene layer thus formed is uniformly coated to the extent of 2 pounds per ream, with a polyamide ink, preferably under their designation CIC 326 W-7 33, Ink H.

The seal formulated with this A layer construction (using the adhesive of Example 10) provides a strong peel strength of 250-275 grams per inch at the polyethylene-polyamide interface.

Example 14 A paper-polyethylene laminate is prepared as in Example 13 but with the polyethylene coating amounting to 12 pounds per ream. The polyethylene layer is then coated with 2 pounds per ream of a curable silicone resin of the following type in a 5% solids solution and then cured for 5 minutes at 150 F.':

Percent SS-4026 16.7 SS-4012 1.7 Toluene a" 81.6

SS-4026 and 55-40-12 are the trade designations for, respectively, a 30% toluene solution of a mixture of high molecular weight linear dimethyl polysiloxane and, as a cross-linking agent, methyl hydrogen polysiloxane and a polysiloxane curing catalyst.

This coating is coated with the adhesive of Example 1a to complete the sea'lable construction. Good peel characteristics are obtained but the strength at the peelable interface is at such a low level (20 grams per inch) that, for such applications as bread end seals, this combination is not as desirable as many 05 those previously described.

Example 15 A paper base sheet is coated With Pentalyn 833 as in Example Series 10 and then this coating is in turn coated with a lacquer composition of 30% polyvinyl stearate in toluene to a coating weight of 2 pounds per ream. The polyvinyl stearate is a granular, white, waxy solid having a melting point of 47-48 C., refractive index 11 of 1.4540 and a viscosity at C. of 558 cps. This coating is in turn coated with the adhesive of Example 1a and the resulting peel strength of this construction is 40-50 grams per inch at the polyvinyl stearate-Pentalyn 833 interface.

Example 16 A paper base sheet is coated with Arochem 455 as in Example Series 11 and then this coating is in turn coated with a 30% toluene solution of chlorosulfonated poly ethylene to a weight of 2 pounds per ream. Chlorosulfonated polyethylene may be obtained under the trade name Hypalon 30 as a white solid having a specific gravity of 1.28. Adhesive applied as in Example 1a results in a sealable construction having a peel strength of -200 grams per inch.

Example 17 This example is to demonstrate the use of a pressure sensitive adhesive in scalable constructions of this invention. The following pressure sensitive adhesive may be coated to a weight of 6 to 10 pounds per ream on A layer constructions prepared as in Examples 1a and 4d on 45 pounds basis weight clay coated and supercalendered sulfite paper:

Percent Vistanex LMMS 63.7 Piccolyte 5-115 31.8 Mineral oil 3.2 Ionol 1.3

Vistanex LMMS is the trade name designation for a polymer of isobutylene having an average molecular weight range of 8,700 to 10,000. It is a tacky, gummy solid having a high degree of cold flow.

Piccolyte S-115 is the trade name designation for a tackifying agent consisting of polyterpene resins derived from hydrocarbon residues of petroleum refining operations having a melting point of 115 C., specific gravity at C. of 0.980 and a 0 acid number.

Ionol is an oxidation inhibitor consisting of di-t-butyl methyl phenol.

This adhesive may either be applied as a hot melt or as a 30% solids solution in cyclohexane.

The nitrocellulose-Parlon construction as in Example la has the same peel strength as in that example but the Parlon-nitrocellulose construction has a peel strength of 30-50 grams per inch as compared with 60-70 grams per inch in Example 4d. In both instances cleavage occurs at the Parlon-nitrocellulose interface.

Example 18 This example is to demonstrate the use of a delayedtack heat activatable adhesive in the constructions of this invention. An adhesive of the following composition may be formulated as described in Example H of U.S. Patent No. 2,746,885 to Holt:

This adhesive is applied at a coating weight of 6 to pounds per ream to paper-A layer constructions prepared as in Examples la, 4d and 7e.

The construction having the Example 1a A layer has a peel strength of 150200 grams per inch, considerably higher than that of Example 1a where a non-delayed-tack adhesive was used. After separation, both A layer coatings are found to be still tacky and occasional fiber pick is experienced, indicating imperfect peel qualities. This is believed to be due to the solubility of the chlorinated rubber coating in the diphenyl phthalate plasticizer of the adhesive which keeps the rubber in a tacky condition, thus causing it to bond more strongly to the nitrocellulose coating from which it must delaminate.

The construction having the Example 4d A layer has a peel strength (50-70 grams per inch) and peel qualities almost identical to that of the construction of Example 4d.

Example 19 This example is to demonstrate the use of a solvent activatable adhesive in the constructions of this invention. An adhesive of the following composition may be formulated by dissolving the solid ingredients in the water:

This adhesive is applied at a coating weight of about 12 pounds per ream to the A layer of Example 7i which has been applied to 60 pound basis weight gumming kraft paper to produce a construction which peels at the polyvinyl alcohol-polystyrene interface at a peel strength of 10-20 grams per inch.

This adhesive formulation has the limitation that it Will not adhere satisfactorily to the Parlon coating of Example Series a, the polystyrene coating of Example Series g and a number of other coatings. It will adhere to the Series 1 Vinsol coating, however. These results point up the necessity of selecting an adhesive which will bond to a satisfactory degree to the A layer.

Example This example is to demonstrate that the choice of base sheet material determines the practical choice of A layer materials. It is also to demonstrate that at very low A layer coating weights peel strength values for a given construction may increase due to imperfections in the delaminating interface caused by strikethrough.

A sealable construction may be prepared as in Example 311 with the modification that the shellac coating is reduced to a coating weight of 1.5 pounds per ream and the Acryloid coating to a coating weight of 1.8 pounds per ream. The peel strength of this construction is so great that, in contrast to Example 3h, the base sheet tears preferentially to A layer delamination.

A similar construction (having the reduced A layer coating weights as above) may be made by substituting glassine as the base sheet in place of the weaker sulfite paper of Example 111. This construction exhibits good peel qualities at the delaminating interface without preferential delamination in the base sheet, since this base sheet has a superior resistance to internal delamination. Of course, the usefulness of a construction of this nature is limited to application to substrates having comparable tear strength.

Before leaving this category it is convenient to illustrate several other heat activatable adhesive formulations which may be employed advantageously with both this and the other types of A layer constructions.

A solvent base adhesive of the following formulation is applied in 35% toluene solution (rather than as a hot melt) at the same coating weight in substitution for theadhesive in the construction of Example 1a to give a heat sealable construction having a peel strength of grams per inch:

Percent Elvax 250 (see Example 1a) 35 Chlorinated parafiin wax (70% chlorine by weight) 55 Immediate petroleum wax (M.P. ISO- F., vis

cosity 6-6.5 centistokes 210 F.) 10

Alternatively an adhesive of the following composition may be formulated by melting and mixing together the ingredients and applied as a hot melt:

Other compositions as described in U.S. Patent No. 3,025,167 to Butler are also suggested.

Another preferable adhesive for use in the construction-s of this invention (such as in Example 1a) is made by substituting for the ethylene-vinyl acetate copolymer (Elvax) in the adhesive composition illustrated in EX- ample 1a (a total of 45%) an ethylene-ethyl acrylate copolymer such as those under the designations X 3488.2 and X 3488.3 (or mixtures of these two). X 3488.2 is a designation for a solvent polymerized ethylene-ethyl acrylate copolymer having a weight content of polymerized ethyl acrylate of 20%, a melt index of 18, a softening point of 53 C., an ultimate tensile strength of 1200 p.s.i., yield strength 450 p.s.i., and a specific gravity of 0.927. X 3488.3 designates solvent polymerized ethylene-ethyl acrylate copolymer having a weight content of polymerized ethyl acrylate of 30%, a melt index of 18, an ultimate tensile strength of 750 p.s.i., yield tensile strength of 250 p.s.i., a specific gravity of 0.932, a softening point of 35 C.

(II) DELAMI'NATIO'N AT ADHESIVE LAYER A INTERFACE Another category of the invention is represented in FIGURE 12, wherein at the left, the seal takes the form of a base sheet 1 bonded to a delaminating layer 2c which is in turn bonded to an adhesive coating 3. The seal is shown aifixed to substrate sheet 30. In this category, layer 20 is selected of a type which will form a weak bond with adhesive coating 3 and a somewhat stronger bond with base sheet 1. In this case, as shown at the right,

when base sheet 1 is pulled from the sheet 30, delamination will readily occur at the interface between layer 2c and adhesive coating 3.

TheA layer of this category of seals is preferably of a coating weight of about 1 to 3 pounds per ream. However, the appropriate coating weight is dictated by the particular type of base sheet employed, its porosity, and absorptive qualities and the other factors discussed under Category 1. Again, the essential factor is to obtain a uniform coating on the base sheet of suflicient thickness to prevent strike-through of the adhesive to the base sheet when it is applied so that the delaminating interface is discontinuous.

Table II shows the delaminating interface bond strength, in accordance with the previously described testing method, for the following exemplary embodiments of this category of the invention.

Example 21 A dampened and supercalendered 35 pound per ream basis Weight gumming kraft paper is coated with a Water solution (15% solids) of the following curable silicone resin composition which is then cured and dried at 250 F. for a short time to give a coating Weight of approximately 0.75 pound per ream.

Percent Syl-Ofi 22 60 22 Catalyst A 7.2 Citric acid 2.8 CMC 7-L 30 Syl-Off 22 and :2 Catalyst A are the trade designations for a water emulsion of a curable organopolysiloxane, 40% nonvolatiles after heating 4 'hours at 105 C. and having a specific gravity of 1.0, and a metal-organic salt curing catalyst of the same specific gravity and 24% nonvolatiles, respectively.

CMC 7- L is the trade designation for carboxy-methyl cellulose of a viscosity range of 6 to 9 cps.

The silicone coating is in turn coated with the adhesive of Example In in the same manner as in that example to complete the construction.

Example 22 Paper of the type described in Example 1a is coated to 3 pounds per ream with clay containing a minor amount of a resin binder by applying the following composition and then air drying to remove the moisture.

Percent Hydrasperse clay 60.9 Pliolite 2000 7.4 Ammonium hydroxide 0.2 Water 31.5

Hydrasperse clay is the trade name for a coating grade, spray dried bead clay having a particle size distribution of 77-80% under 2 microns and 36% above microns. This clay analyzes approximately 45% silica and 38% alumina with minor amounts of iron oxide and titanium dioxide.

Pliolite 2000 is a product comprising a Water dispersion (41% solids) of a 50/50 monomer ratio styrene-butadiene copolymer. Average particle size is 1300 Angstroms and specific gravity 0.98 (latex solids).

The dried clay coating is in turn coated with the ad hesive of Example 1a in the same manner as that example to complete the construction.

Example 23 Paper of the type described in Example In is coated to 3 pounds per ream with sodium silicate. The sodium silicate is applied as a 30% water solution and air dried. The thermoplastic adhesive of Example In is then applied to this coating to complete the construction.

Note from Table II that imperfect peel qualities are obtained with this particular construction. Substitution of a stronger base sheet as in Example 20 will provide more satisfactory results.

Example 24 Supercalendered and clay coated 45 pounds per ream basis Weight sulfite paper is extrusion coated with 5 to '7 pounds per ream of polypropylene having a nominal melt flow of 60 and specific gravity of 0.905. This type of 20 polypropylene may be obtained under the trade designation Profax 6130.

The polypropylene layer is then coated to 10 pounds per ream with the following heat activatable adhesive formulation which will bond well to many of the more important substrates such as waxed paper, cellophane, Saran and polyethylene:

Percent Elvax 250 (see Example 1a) 30 Microcrystalline Wax (MrP. 145150 F, viscosity -100 cps. 210 F.) 55

Piccolyte S-115 (see Example 17) 15 This formulation is prepared by heating the ingredients and blending them together in a hot melt. Since this adhesive is chosen for its peelability from polypropylene, this construction has the inherent limitation that it does not bond well to a polypropylene substrate. Other constructions of this invention are thus more suitable for application to a polypropylene substrate.

(III) DELAMINATION AT THE BASE SHEET- LAYER A INTERFACE Another general category of the invention is shown in FIGURE 13, where, at the left, the seal takes the form of base sheet 1, Weakly bonded to a delaminating layer 2d, which is, in turn, more firmly bonded to an adhesive layer 3a. The seal is shown attached to substrate sheet 30 by means of the adhesive layer 3a. As shown at the right in this figure, delamination occurs at the interface between base sheet 1 and layer 2d when the base sheet is pulled from sheet 30.

While various combinations of base sheet materials and layer A coatings may be used to achieve the desired low bond interface therebetween, the strength of the delaminating interface bond in this category depended also to a large extent upon the conditions under which the A layer is applied to the base sheet. Thus, the conditions of application may be selected for a given construction to give the specific bond strength which is desired.

It is particularly desirable with this particular type of construction that the A layer be sufiiciently weak so that it may easily be torn from the substrate material which it binds together after the base sheet is peeled therefrom. Thus, preferably, the A layer is as thin as possible while still providing a continuous coating. For the most desirable tear qualities, the A layer is preferably below 8 pounds per ream basis weight and desirably 5 pounds per ream. But, again, the optimum amount depends primarily on the material used for the A layer and to some degree upon the type of base sheet and adhesive. Also, the other factors previously discussed with regard to strikethrough may apply.

It should also be noted that in this particular embodiment, when the A layer is a thermoplastic substance and the adhesive is heat activatable, it may be desirable to restrict the temperature at which the seal construction is heated for application to a substrate since at high temperatures the thermoplastic A layer may tend to bond more strongly to the base sheet to thus eliminate the delaminating interface.

Example 25 To exemplify this category, a polyethylene film may be weakly bonded to a base sheet of 35 pounds per ream basis Weight supercalendered bleached sulfite paper, coated with approximately 0.5-0.7 pound per ream of starch sizing, in the following manner to give a delaminating interface bond strength of 60 to grams.

The paper is passed under the film extrusion head of a conventional polyolefin extrusion coating apparatus and a film of low density polyethylene (0916-0920) at a temperature of 575 F. is extruded therefrom onto the adjacent face of the paper sheet with an air gap between the extrusion head and the paper of 3 /2 inches. The coating speed is 650 feet per minute and the weight of the coating layed down is pounds per ream.

The polyethylene is then coated with pounds per ream of the adhesive and in the manner described in Example 1a.

Example 26 To demonstrate the manner in which the variables may be controlled to give the desired peel strength, very similar peel strength results may 'be obtained as in Example 25 when 40 pounds per ream natural kraft paper without any starch sizing (providing a rougher surface to which the polyethylene bonds more readily), is coated in the same manner with the same density of polyethylene but at a polyethylene temperature of 560 F. (the lower the temperature, the lower the resulting bonding power) and at a coating speed of 720 feet per minute (the higher the speed, the less time the polyethylene is exposed for oxidation in the air gap and the less oxidation at the film surface, the poorer the resulting bond). Thus, the various factors determining the resulting bond may be set off against each other to achieve the desired bond. The length of the air gap and the density of the polyethylene may also be varied to control the bond strength.

Although perhaps to be completely logical, the embodiment of Example 25 should appear under Category I, previously described, on the basis that the starch size coating on the paper can be considered to be a first coating of the A layer and the polyethylene layer the second, it and the following suggested embodiments are inserted at this point for ease of description.

Some potential A layer material, such as polyester, nylon, polystyrene or polypropylene resins, bond so poorly to base sheet material such as paper that in many or all instances with a particular combination, it is necessary to prime the surface of the base sheet with some mutually attractive substance compounded to create a bond with the paper with the proper tenacity and delaminating properties. For instance, when paper is the base sheet, it is usually necessary to prime it with a light coating (forinstance, 0.5 pound per ream) of a primer such as a composition of starch and a copolymer of vinyl acetate and vinyl chloride before coating it with an A layer constituting polyester or polystyrene. The percentage of starch and copolymer in the primer may be adjusted to give the degree of peelabili-ty desired. Except for a paper base sheet having a very rough surface, the same type of primer treatment is generally required when the A layer is polypropylene or nylon.

(1V) DELAMINATION WITHIN A SINGLE COATING COMPRISING LAYER A An alternative type of construction is illustrated in FIGURE 14, where, at the left, a seal consisting of base sheet 1, intermediate delaminating layer 2e and adhesive coating 3, is shown adhered to the surface of substrate sheet 30. In this case, delaminating layer 2e consists of material having a weak internal bond strength which is adhered in some fashion to the adhesive coating 3 on one side and base sheet 1 on the other. Layer 2e may take the form of a coating of finely divided particles such as clay weakly held together by a binder such as latex which also bonds this layer to adhesive layer 3 and base sheet 1. When base sheet 1 is pulled away from the sheet 30 as at the right in FIGURE 10, delamination occurs in layer 2e before enough stress is placed on base sheet 1 or sheet 30 to tear or otherwise disrupt them.

Layer 2e is necessarily a continuous coating so that there is no significant amount of adhesive strikethrough to the base sheet. The amount of material required to form this coating depends on the type of base sheet and adhesive and the other factors previously discussed in conjunction with the Category I constructions. Generally a satisfactory coating 2e is obtained with an amount of 4 to 5 pounds per ream of the particular material used. The following examples are illustrative of constructions of this type.

Example 27 A base sheet as in Example 1a is coated to a coating weight of 5 pounds per ream with a chemically stable butyl rubber emulsion (55% solids) containing 2.5% of an anionic emulsifier and 400 p.p.m. formaldehyde to inhibit microorganism growth, the butyl rubber having 1.5 to 2 mole percent unsaturation, specific gravity 0.96, and a particle size range of 90% between 0.051.0 micron (average 0.5). A latex of this description is available under the designation Latex -21.

The latex layer is coated with the adhesive of Example In to provide a sealable construction which will delaminate within the butyl rubber layer with a peel strength of 250-300 grams per inch.

Example 28 A base sheet as in Example 1a is coated to a weight of 5 pounds per ream with a white pigment in a minor amount of binder. Titanium dioxide pigment containing approximately 3.5% nitrocellulose as a binder may be thinned to a desirable coating consistency with isopropyl alcohol, applied and then dried. This coating is in turn coated with the adhesive of Example 1a to give a sealable construction having a peel strength of 10-20 grams per inch. Delamination occurs not only internally of the pigment layer but also both at its interface with the base sheet and with the adhesive.

The low bond strength of this construction renders it generally unsatisfactory for use in bread end seals but it may be used in other applications where scuff resistance is not essential. To increase the delaminating strength a stronger bonding agent may be used or the proportion of the present bonding agent may be increased.

Example 29 A paper-first A layer coating construction is made as in Example Series 12 but with a Beckacite 1126 coating increased to 5 pounds per ream. The adhesive of EX- ample 1a is then applied to create a sealable construction which will delaminate within the Beckacite layer at a peel strength of 75 to grams per inch. Similarly, where the Vinsol coating of Example Series 6 is substituted (at 5 pounds per ream) for the Beckacite 1126 layer in this construction, a construction is obtained which will delaminate within the Vinsol coating at a peel strength in the range of 80-120 grams per inch. This example further illustrates that the same A layer material may be used to produce constructions falling within more than one of the categories defined for description of the invention.

ALTERNATIVE POSITIONING OF THE A LAYER An alternative positioning of the A layer for seal constructions embodying the principles of this invention, equally applicable to the foregoing types of A layer combinations, is the placement of the A layer on the substrate rather than on the base sheet. This particular method of seal construction has the advantage of permitting the use of commercially available adhesive coated sealing sheets.

For example, a wrapping sheet may be uniformly coated on one side with one of the A layers previously described such as a first coating of nitrocellulose and a coating thereover of chlorinated rubber as applied to the base sheet in Example 1a. The wrapping sheet may then be wrapped around the article to be packaged, such as bread, with the A layer to the outside. Then, in the well known manner of the prior art, an adhesive coated sealing sheet or label, such as the 48 lbs/ream sheet of paper described in Example 1a coated with 15 pounds per ream of the heat activatable adhesive of Example 1a, may be heated and applied to marginal portions of the 23 wrapper as at the end folds. The seal thus constructed may be peeled off at the A layer without tearing either the label or the wrapper so the wrapper is left intact for reclosing. Of course, in this instance the adhesive does not remain on the wrapper as in the previous examples but is peeled off with the label.

The A layer coating need only cover the area of the substrate to which the sealing sheet is to be applied and need not cover the entire sheet as in the foregoing example. It is preferable, in using this type of seal construction for closing containers not constructed of flexible material and for other uses, to cover with the delaminating layer only the portion of the container, etc. to which the sealing sheet is to be applied.

SHEAR STRENGTH IN THE PLANE OF THE SEAL An important criterion of a good seal is adequate strength in shear in the plane of the seal. Where, for instance, a scalable sheet is afiixed to the end folds of a package, the strength of the laminate thus formed must be sufiicient to prevent pulling out of the end folds when forces are applied to the end folds to pull them outwardly in the plane of the seal as when the package is handled, squeezed, etc. The optimum in shear strength in the plane of the seal is where this shear strength is greater than the tensile strength of the wrapping sheet or other substrate or the base sheet of the seal so that failure occurs in the package rather than at the bonds holding the seal to the package. While shear strength in the plane of the seal is a function of the area of the seal, it has been found that, even for a very small seal area, the tensile strength of most substrates and base sheet materials is less than the shear strength of the seals of this invention in almost every case.

Tests with various of the seal constructions to compare the strength in shear of the seals relative to the tensile strength of the base sheet and the substrate are illustrated in Table HI. Also shown are the tensile strengths of various substrates and base sheets for comparison.

Tensile strengths were determined by the use of a Schopper type tensile tester. For the tensile tests strips of the various base sheet material and substrates were loaded in tension to the point of failure to determine their tensile strength. This test was also used to determine the strength in tensile of the substrates and base sheets relative to the shear strength of a seal area of 1 inch by 2% inches by preparing test strips composed of a base sheet strip and a paper substrate strip, .both 1 inch wide and overlapped for a distance of 2% inches, the overlapped area of the base sheet strip being provided with coatings or layers in accordance with the present invention to form a seal construction and heat sealed to the overlapped area of the substrate strip. In all but one instance, failure occurred in the base sheet or substrate, itself, rather than in the seal therebetween.

Thus, for most practical applications of the seals of this invention, the particular choice of the A layer is not critical in developing adequate seal shear strength. Rather, the choice of the A layer is dictated by the desirable amount of peel strength as discussed previously.

The seal constructions of this invention find application in containers or packages of all types. While they are particularly adapted to packaging perishable food products, such as breads, meats, breakfast cereals, flour, cookies and the like, the constructions may be used on packages containing almost any packagable item. It has previously been pointed out that the scalable constructions of this invention may be prepared in the form of pricing labels, sealing tapes and the like useful for sealing to closures. However, in addition, they may find utility in many other applications where it is useful to temporarily affix a supplemental sheet of material to a substrate.

It is apparent from the examples given that many different materials may be utilized in the manufacture of seals of the invention. Also, new materials such as synthetic resins and polymers are constantly being developed and made commercially available, many of which undoubtedly will be found adaptable as components of adhesive and A layer coatings in the practice of this invention.

Having thus disclosed the invention, I claim:

1. A label for heat scaling to flexible packaging material comprising, a paper base sheet, a delaminating layer adhered to and covering at least :a portion of one side of said base sheet and a heat activatable adhesive coating adhered to and coating at least a major portion of said delaminating layer in an amount between 8 and 20 pounds per ream of said base sheet, said delaminating layer having an interface located intermediate said base sheet and said heat activatable adhesive coating and characterized by a mechanically disruptable bond having a peel strength greater than 20 grams per lineal inch, measured by pulling said base sheet back over itself at a rate of 12 inches per minute, but less than the internal delaminating strength of said paper base sheet, and by being nonfusible at the activation temperature of said adhesive coating, whereby when said label is heat sealed to a flexible packaging material said base sheet may be peeled from said adhesive coating at said delaminating interface without tearing said label or said flexible packaging material, said base sheet having an adhesive-free margin to provide a pull tab.

2. A reclosable food package comprising a flexible wrapper about a food product and closed over a portion of said product by folds, a sealing sheet overlying said folds and heat sealed thereto by a delaminable adhesive bond between said seating sheet and said folds thereby enabling the sealing sheet to be peeled off said folds without tearing said folds or said sealing sheet, whereby, when the sealing Table III SHEAR AND TENSILE MEASUREMENTS IN PLANE OF SEAL Tensile Tensile Type of A layer in Seal (or Material) Strength Strength WMD CMD Type of Rupture (or Shear) (or Shear) Lbs/In. Lbs/In.

Nitroccllulose-Parlon (of Example 1a), 22. 4 8. 3 Base Sheet Failure. Iolyethylene-Polyamide Ink (of Ex- 12.8 12.0 (WMD) Base Sheet Failure. (CMD) Subample l3). strate-polyethylene failure. Weakly Bonded Polyethylene (of 24. 2 18.0 (WHD) Base Sheet Failure. (CMD) Sub- Example 25). strate-polyethylene failure. Ioliygethylene-Silicone (of Example 10.4 17.6 Polyethylene-Substrate Failure.

1 Silicone (of Example 21) 22.4 18. 4 (WMD) Base Sheet Failure. (Ch ID) Seal Failure in Shear.

Polyethylene 3. 5 2. 4 Polypropylene 4. 6 7. 5 Tango Cellophane 17. 5 8. 2 Paper (48 lbs/ream sulhte) 22. 4 7.0

sheet is peeled ofI and a portion of the product is removed, the package can be reclosed by said folds, said delaminable adhesive bond comprising a heat sealed thermoplastic adhesive layer adjacent said folds and at least one delaminating layer intermediate said adhesive layer and said sealing sheet and having an interface located intermediate said adhesive layer and said sealing sheet and characterized by a mechanical disruptable bond having a peel strength less than the internal delaminating strength of said Wrapper and said sealing sheet and by being nonfusible at the activation temperature of said adhesive.

3. A reclosable flexible bread package comprising, in combination, a flexible packaging sheet secured around the lateral surfaces of a loaf of bread, overlapped end folds at an end of the loaf and a flexible end label comprising a paper base sheet having at one face thereof a heat-sealed resinous adhesive coating bonding said label to said end folds, said adhesive coating being solid and nontacky at room temperature, and .a delaminating layer intermediate said heat-sealed coating and said paper base sheet bonding said heat-sealed adhesive coating to said base sheet, said delaminating layer being of material having a rupturable bond located intermediate said base sheet and said heat sealable adhesive coating and which is mechanically disruptable upon peeling of said base.

sheet from said end folds and which is nonfusible at the activation temperature of said adhesive coating, said rupturable bond being weaker than the bond strength of said heat sealed adhesive coating to said end folds to enable peeling of said paper base from said closure without fiber tear, whereby after peeling of said end label the end folds can be readily opened to enable removal of a portion of the bread and the package can be reclosed over the remaining portion of the bread to further protect the same.

4. A flexible sheet for heat sealing to a substrate comprising, a flexible base sheet, a delaminating layer adhered to and covering at least a portion of one side of said base sheet and a heat-activatable adhesive coating adhered to and coating at least a major portion of said delaminating layer, said laminating layer having an interface located intermediate said base sheet and said heat-activatable adhesive coating and characterized by a mechanically disruptable bond weaker than the internal delaminating strength of said base sheet and by being nonfusible at the activation temperature of said adhesive coating, whereby when said sheet is heat sealed to a substrate said base sheet may be peeled from said adhesive coating at said delaminating interface without tearing said base sheet or said substrate.

5. A sheet as in claim 4 and wherein said adhesive coating comprises a composition containing between about and of a petroleum wax having a melt point between F. and F. and between about 50% and 30% of a high molecular weight thermoplastic copolymer of ethylene and vinyl acetate, said copolymer having a weight content of polymerized vinyl acetate of between 20% and 35%.

6. A sheet as in claim 4 and wherein said delaminating layer comprises a continuous coating of a composition including a cured organopolysiloxane resin as the major component thereof.

7. The flexible sheet of claim 4 wherein said flexible base sheet is paper.

8. The flexible sheet of claim 4 wherein said delaminating layer comprises different film forming coatings, one bonded to the base sheet and another bonded to the heat-activatable adhesive coating, and which are bonded together along said interface.

9. The package of claim 2 wherein the peel strength at said interface having a mechanical disruptable bond is between about 20 and about 200 grams per lineal inch, measured by pulling said sealing sheet back over itself at a rate of 12 inches per minute.

10. The package of claim 9 wherein said peel strength is between about 20 and about 80 grams per lineal inch.

11. The sheet of claim 4 wherein said delaminating layer is a material selected from the group consisting of natural resins, modified natural resins, modified natural polymers, and synthetic polymers.

References Cited by the Examiner UNITED STATES PATENTS 2,681,732 6/ 1954 Brady.

2,838,168 6/1958 Schwarz.

2,883,044 4/ 1959 Kendrick.

2,984,342. 5/ 1961 Smith.

3,038,597 6/1962 Brady.

3,076,588 2/ 1963 Conway et al 229-35 A. LOUIS MONACELL, Primary Examiner. RAYMOND N. JONES, HYMAN LORD, Examiners. 

2. A RECLOSABLE FOOD PACKAGE COMPRISING A FLEXIBLE WRAPPER ABOUT A FOOD PRODUCT AND CLOSED OVER A PORTION OF SAID PRODUCT BY FOLDS, A SEALING SHEET OVERLYING SAID FOLDS AND HEAT SEALED THERETO BY A DELAMINABLE ADHESIVE BOND BETWEEN SAID SEATING SHEET AND SAID FOLDS THEREBY ENABLING THE SEALING SHEET TO BE PEELED OFF SAID FOLDS WITHOUT TEARING SAID FOLDS OR SAID SEALING SHEET, WHEREBY, WHEN THE SEALING SHEET IS PEELED OFF AND A PORTION OF THE PRODUCT IS REMOVED, THE PACKAGE CAN BE RECLOSED BY SAID FOLDS, SAID DELAMINABLE ADHESIVE BOND COMPRISING A HEAT SEALED THERMOPLASTIC ADHESIVE LAYER ADJACENT SAID FOLDS AND AT LEAST ONE DELAMINATING LAYER INTERMEDIATE SAID ADHESIVE LAYER AND SAID SEALING SHEET AND HAVING AN INTERFACE LOCATED INTERMEDIATE SAID ADHESIVE LAYER AND SAID SEALING SHEET AND CHARACTERIZED BY A MECHANICAL DISRUPTABLE BOND HAVING A PEEL STRENGTH LESS THAN THE INTERNAL DELAMINATING STRENGTH OF SAID WRAPPER AND SAID SEALING SHEET AND BY BEING NONFUSIBLE AT THE ACTIVATION TEMPERATURE OF SAID ADHESIVE.
 4. A FLEXIBLE SHEET FOR HEAT SEALING TO A SUBSTRATE COMPRISING, A FLEXIBLE BASE SHEET, A DELAMINATING LAYER ADHERED TO AND COVERING AT LEAST A PORTION OF ONE SIDE OF SAID BASE SHEET AND A HEAT-ACTIVATABLE ADHESIVE COATING ADHERED TO AND COATING AT LEAST A MAJOR PORTION OF SAID DELAMINATING LAYER, SAID LAMINATING LAYER HAVING AN INTERFACE LOCATED INTERMEDIATE SAID BASE SHEET AND SAID HEAT-ACTIVATABLE ADHESIVE COATING AND CHARACTERIZED BY A MECHANICALLY DISRUPTABLE BOND WEAKER THAN THE INTERNAL DELAMINATING STRENGTH OF SAID BASE SHEET AND BY BEING NONFUSIBLE AT THE ACTIVATION TEMPERATURE OF SAID ADHESIVE COATING, WHEREBY WHEN SAID SHEET IS HEAT SEALED TO A SUBSTRATE SAID BASE SHEET MAY BE PEELED FROM SAID ADHESIVE COATING AT SAID DELAMINATING INTERFACE WITHOUT TEARING SAID BASE SHEET OR SAID SUBSTRATE. 